The use of a spring connected between a door and its associated jamb to automatically return the door to its closed position is well known, and the use of a pneumatic or hydraulic dashpot to retard the closing speed of the door to prevent it from slamming against the door frame is also well known.
Hydraulic door closers have the added advantage of also retarding the opening speed of the door and thus inerehently preventing excessive opening of the door by wind gusts or the like. Moreover, hydraulic door closers have generally provided a smoother movement of the door during closing rather than the bouncy movement commonly associated with pneumatic door closers.
On the other hand, hydraulic door closers have an inherent leakage problem, and leakage of the hydraulic liquid not only adversely affects the operation of the door closer, but it is also messy. Moreover, hydraulic door closers are generally more expensive to manufacture than are pneumatic door closers.